Because of the extremely high corrosion that conventional metal materials exhibit with respect to aluminum and other nonferrous metals at typical processing temperatures, areas of contact between the nonferrous metal and the permanent mold must be treated with so-called mold release agents so that trouble-free operation is possible. Especially but not exclusively in the case of pressure casting processes, which are characterized by the use of high temperatures and pressures, a wide variety of requirements, which are listed below, are placed on the mold release agents employed. Thus, the mold release agent must promote metal flow, which results in uniform filling of the permanent mold, while simultaneously serving to improve the demoldability of the cast parts. Furthermore, the mold release agent serves to avoid residues on the permanent mold that can result in inaccuracies in the mold. Excessive gas formation during decomposition of the mold release agent, which would result in porosity of the molded parts, must not occur while the material is being poured into the permanent mold. Lastly, the mold release agent must not contain any hazardous or toxic substances. The quality of the mold release agent is measured by how well it meets these requirements.
Boron nitride (BN), which has a crystal structure similar to that of graphite, is a material that has long been known and used in mold release agents. Like graphite, it has low wettability with respect to many substances, such as silicate melts and also metal melts.
For this reason, there are many studies on non-stick coatings based on boron nitride in order to use it for casting processes. However, the problem with this use is that it does not succeed in permanently applying boron nitride to molds, especially those of a complex nature. A method for permanently applying a temperature-stable, corrosion-resistant mold release layer is described in DE 198 42 660 A1. In this process, a boron nitride powder is applied to the surface of a permanent mold by means of electrostatic coating.
Attempts have also been made to produce binders with inorganic bases incorporating boron nitride. U.S. Pat. No. 6,051,058 describes the production of boron nitride protective coatings with thicknesses from 0.2 to 0.7 mm on refractory materials for the continuous casting of steels. Here, boron nitride on the order of magnitude of 20 to 50% by weight is bound to the refractory material with the aid of high temperature binders in the form of an aqueous coating solution based on metal oxides in the group consisting of ZrO2, zirconium silicates, Al2O3, SiO2, and aluminum phosphates.
To suppress the wear and corrosion of materials, an anti-abrasion layer is known from DE 101 24 434 A1, in which functional materials are dispersed in a binder matrix. This so-called functional coating consists of an inorganic matrix phase, which consists at least primarily of a phosphate, and a functional material embedded therein, which may be, for example, a metal, graphite, a hard material, a dry lubricant, an aluminum oxide, a silicon carbide, etc. This document also describes a method for producing this functional coating, wherein a functional material in powder form is dissolved in a liquid component, which may be water for example, and to which phosphoric acid is added in order to produce a phosphate. A matrix solution of such a composition, with the liquid component and the phosphate, may also be called a gel because of its consistency. After a material has been coated with this matrix solution, the material undergoes a heat treatment so that an adherent functional coating forms on the base material.